This invention relates to an apparatus and method for making foamed liquid cleaning solutions. In a specific application the foamed liquid solutions are used for cleaning the interior surfaces of rigid vessels, such as heat exchangers.
There are several types of vessels, such as shell and tube heat exchangers, in which water is used as a heat transfer fluid. Usually the water will contain dissolved materials, such as metal oxides, inorganic salts, and the like. Over a period of time the dissolved materials will form scale deposits on the inside surfaces of the tubes, which make up the tube bundle in the vessel. These scale deposits must be removed periodically to permit the heat exchanger to operate effectively.
A common technique for removing scale deposits from the interior surfaces of tubes in a heat exchanger is to circulate, under pressure, a foamed cleaning solvent through the scale-coated passages. A typical apparatus and method used in a foam cleaning operation is described in U.S. Pat. No. 3,212,762. The cleaning solvent employed in this operation is an aqueous solution of hydrochloric acid. The composition also contains ammonium bifluoride, to help remove scale containing silicate compounds, and a suitable foaming agent.
In a typical operation, the cleaning composition is premixed and held in a tank truck. From the tank truck the cleaning composition is pumped into aerator tanks in a foam generator. A pair of cylindrical sparger tubes, which are fabricated of an aluminum silicate material, are positioned in each aerator tank. In operating position the sparger tubes are partially submerged in the cleaning liquid. The foam composition is generated in each aerator tank by forcing compressed air through the pores in each sparger tube. From the aerator tanks the foam composition flows into an adjacent tank, and is forced through a discharge line into the vessel to be cleaned.
The foam generator described above has several disadvantages from the standpoint of commercial use. One disadvantage is that the sparger tubes disintegrate in a relatively short time, due to attack by the ammonium bifluoride on the aluminum silicate composition in the sparger tubes. Another disadvantage is that the compressed air used to foam the cleaning solvent is sometimes contaminated with entrained oil. During a foaming operation the oil will coat the inner surfaces of the spargers. This results in a fairly rapid breakout of the foam, i.e. most of the foam composition reverts back to a liquid phase. Another disadvantage of this apparatus is that the discharge line, from the foam generator to the vessel being cleaned, should be a fairly large diameter line. If a small diameter line is used, i.e. less than about 4 inches, the resulting back pressure against the spargers (above 15 psig) may cause the spargers to burst.
Another disadvantage of this apparatus is that the rate of air flow to the spargers cannot be stepped up to get a higher ratio of foam to liquid in the cleaning composition. If the air velocity is increased the air stream will pass through the pores of the spargers in the form of jets. These jets of air thus form large bubbles in the liquid and thereby inhibit the foam dispersion. Another disadvantage is that this apparatus generally requires a recycle pump. The pump is necessary for recirculating of excess liquid, caused by foam breakout, from the holding tank back through the spargers in the aerator tanks.